The procedure was associated with a 21 percent relative reduction in incidence and a 26 percent relative reduction in death, although the mortality benefits were limited to cancers of the distal, not the proximal, colon, Robert Schoen, MD, MPH, of the University of Pittsburgh Medical Center, and colleagues reported in the New England Journal of Medicine and at a session here at Digestive Disease Week.

Flexible sigmoidoscopy doesn't scan the entire colon, particularly its furthest reaches, but researchers estimate that a majority of cancers arise in the sigmoid colon.

Yet its effectiveness as a screening tool has not been settled. Earlier trials, including one from Italy, found a lower incidence of colorectal cancer with the screening tool, while a Norwegian trial found no benefits.

Between 1993 and 2001, a total of 154,900 patients, ages 55 to 74, were assigned to screening with flexible sigmoidoscopy with a repeat screen at 3 or 5 years, or to usual care, which involved some patients having a colonoscopy or a sigmoidoscopy or no screening at all.

Of the 77,445 patients who were randomly assigned to screening, 83.5 percent had a baseline sigmoidoscopy and 54 percent were screened at 3 or 5 years.

The researchers found that the incidence of colorectal cancer after a median follow-up of 11.9 years was 11.9 cases per 10,000 person-years in the intervention group compared with 15.2 cases per 10,000 person-years in the usual-care group.

With regard to mortality, there were 2.9 deaths from colorectal cancer per 10,000 person-years in the intervention group compared with 3.9 per 10,000 in the usual-care group, corresponding to a 26 percent relative risk reduction with sigmoidoscopy.

Mortality from distal colorectal cancer was significantly reduced, but death from proximal colorectal cancer was unaffected, they reported.

The proximal colon presents more of a challenge, they noted, because of limitations in bowel preparation, a greater prevalence of advanced serrated adenomas in this part of the colon, and other biologic differences, including a greater incidence of BRAF mutation.

"Although our protocol was associated with a reduction in the incidence of proximal colorectal cancer, presumably because of the detection and removal of precursor adenomas that would have otherwise progressed to cancer, it apparently did not succeed in identifying and successfully removing a proportionally greater number of precursor lesions destined to develop into fatal colorectal cancers," they wrote.

Men appeared to have a mortality advantage over women with sigmoidoscopy, with a 34 percent relative reduction in risk compared with a reduction of 13 percent for women, they added.

Men also had more false positives than women (20 percent versus 13 percent), but the researchers said some of these may have been attributable to false negative results on colonoscopy.

Schoen and colleagues also cautioned that a large proportion of the usual-care group (47 percent) had either a flexible sigmoidoscopy or colonoscopy, which "probably reduced the difference in mortality and incidence between the two groups."

In an accompanying editorial, John Inadomi, MD, of the University of Washington in Seattle, noted that the degree of contamination in the usual-care group was far greater than the anticipated 15 percent and likely narrowed the gap in incidence and mortality reduction.

"[The] real question for U.S. clinicians is whether we are prepared to refocus attention on a screening strategy that has been likened to performing mammography on one breast. National guidelines include flexible sigmoidoscopy as a recommended screening strategy; however, use of this test has significantly decreased in the United States," he said.

Inadomi said he sees little reason for that decrease, especially since the quality of evidence supporting colonoscopy is inferior to that supporting sigmoidoscopy.

"Although case-control and prospective cohort studies have been performed, no randomized clinical trial proving that colonoscopy can reduce cancer mortality has yet been published," he wrote, adding that some of these studies have shown that colonoscopy doesn't reduce mortality from proximal colorectal cancer.

The overall benefit from colonoscopy in the population-based studies that exist is similar to the effect of flexible sigmoidoscopy seen in the PLCO trial, he said.

Inadomi added that some patients would prefer alternate screening over colonoscopy, which should also be taken into account when recommending a screening, and healthcare reform in the U.S. has led to a renewed focus on cost-effectiveness in evidence-based resource use.

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